Modern control systems with valves controlling several work elements from a common pressure fluid source are more and more often constructed to receive a so-called load-sensing function. Upon load-sensing, the work elements being activated are scanned and receive a volume flow from a pressure fluid source through a pressure conduit to the work element requiring the highest pressure. In a fully hydraulic solution this is obtained by using several different check valves, The highest sensed pressure signal is returned to the pressure fluid source and controls the source such that it outputs the return pressure value increased by a value required for the valve to be able to control the volume flow with a pressure drop of usually about 25 bar. Thus, the work element requiring the highest pressure is, like the other work elements, subjected to an unnecessary energy loss. In electrohydraulic control systems the highest pressure can be chosen and measured with an electric pressure sensor. Each work element must then have at least one sensor for each force direction, such as two sensors for double-acting cylinders and motors and also at least one pressure sensor positioned on the supply conduit between the pressure fluid source and the valves of the work element. Since electric pressure sensors are considered to involve higher costs and lower reliability than fully hydraulic devices based on check valves, the fully hydraulic alternative has up to now been predominant.
Load-sensing systems and electrohydraulic systems exist substantially in those cases where the requirements are comparatively high, since the cost for the control system is higher than for the simpler conventional control systems.
The load-sensing systems have neither developed as rapidly nor have they received the large market share as many have expected. The explanation for this is probably that the advantages have not been considered to balance the increased cost.
Electrohydraulic control systems have rapidly been developed and continuously take larger and larger market shares. High demands on automatization, increased supervision and security press the development towards electrohydraulic systems having position sensors and ability for position control. Microprocessors and improved pressure and position sensors today make advanced control techniques possible with respect to both function and cost. A relatively rapid and intensive development of electrohydraulics has, however, mainly been directed towards improvements of the conventional hydraulic components and control principles with electrohydraulic solutions rather than creating new control principles and components adapted to the completely new conditions that the access to microprocessors and sensors provides.